Walking to school on a cold morning:
Elder offspring: I’m going to steal your warmth!
Dr. Free-Ride: Oh really?
Elder offspring sticks hands in Dr. Free-Ride’s coat pockets, where Dr. Free-Ride’s hands are.
Elder offspring: Brrr! Your hands are really cold!
Dr. Free-Ride: Yes, they are. Mwah ha ha!
Elder offspring: I’m still going to steal your warmth!
Dr. Free-Ride: My dear, given that in this universe heat flows from hotter objects to cooler ones, I’m pretty sure it is I who will steal your heat.
* * * * *
Toweling off after a recent swimming lesson and noting that the indoor air temperature was significantly lower than the water temperature in the heated pool:
Younger offspring: Please dry me off quickly. I’m freezing!
Dr. Free-Ride: Many are cold; few are frozen.
Younger offspring: Brr-brr-brr-brr.
Dr. Free-Ride: Oh, all right. Hey, do you know why you get cold when you get out of the pool?
Younger offspring: Because the heat-lamp isn’t turned on.
Dr. Free-Ride: Sure, but you’re usually cold right out of the pool even during the summer, when it’s pretty warm in here.
Elder offspring: Because we’re wet.
Dr. Free-Ride: Uh huh. And why does being wet make you cold? You were wet while you were in the pool swimming, and you weren’t cold then.
Younger offspring: Hmm …
Dr. Free-Ride: You remember states of matter, right?
Younger offspring: Yeah. The water in the pool is liquid.
Dr. Free-Ride: And the liquid water that clings to you when you come out of the pool — at least, what we don’t towel off of you — is going to evaporate, which means —
Younger offspring: It’s going to make storm clouds?
Dr. Free-Ride: Indoors? That would be surprising.
Elder offspring: It would be pretty cool, though.
Dr. Free-Ride: Think about how we can turn liquid water into water vapor at home in the kitchen.
Elder offspring: We heat it!
Dr. Free-Ride: That’s right. So what do you think happens when the drops of water on you evaporate?
Elder offspring: It uses heat from our bodies to turn the water from a liquid to a gas!
Dr. Free-Ride: Yup. That’s also why our bodies sweat. Evaporating sweat is a way to move excess heat.
Younger offspring: So evaporating water steal our warmth?
Dr. Free-Ride: That’s thermodynamics for you.